1. Field of the Invention
The present invention relates to a dipole component triggered by a breakover voltage, i.e. a component having the function of a Shockley diode that is turned on when the voltage becomes higher than a predetermined voltage. The present invention more particularly relates to such a component in which the breakover voltage and breakover current can be accurately determined upon fabrication. The invention provides such a component in which the breakover current is low while having reduced sensitivity to spurious triggering associated with voltage edges (dV/dt triggering).
2. Discussion of the Related Art
Such a component is, for example, useful in a gas-lighter circuit such as the circuit illustrated in FIG. 1. This circuit includes, between supply terminals A and B, corresponding for example to the 220-V, 50-Hz a.c. voltage of the mains, a resistor Rs, a rectifying diode Dr, a switch S, a capacitor C and the primary of a high frequency transformer T. The secondary of the transformer comprises windings L1 and L2 connected to lighters 1 and 2, respectively, for example associated with two fires of a gas cooker. A circuit comprising a parallel thyristor Th, which is head-to-tail connected to a diode D, is connected between the input terminal of capacitor C and terminal B. An avalanche diode Z is disposed between the gate and the anode of thyristor Th.
The operation of the circuit will be described with relation to FIG. 2. In FIG. 2, the curve 10 represents the voltage between terminals A and B which is an a.c. voltage having a peak value Vp. Once switch S is turned on, at the beginning of a positive half-period, capacitor C begins to be loaded until the avalanche voltage VZ of the avalanche diode Z is reached. Then, a current tends to flow in the gate-cathode junction of thyristor Th. When this current reaches value I.sub.B0, the thyristor Th becomes conductive in a low impedance state and capacitor C discharges in thyristor Th, then through diode D, thereby producing a discharge current oscillating at a high frequency from capacitor C to thyristor Th and in the parallel, head-to-tail connected diode D. This current oscillation is converted by the transformer to windings L1 and L2, which causes sparks to occur in the lighters 1 and 2.
This circuit requires strict conditions for the triggering of thyristor Th. Indeed, once the avalanche voltage VZ of diode Z is reached, a sufficient current I.sub.B0 to trigger the thyristor Th should be able to flow. This current I.sub.B0 is provided by the rectified supply voltage. The maximum value of this current is determined by equation: EQU RsI.sub.B0(MAX) =Vp-VZ,
where Vp designates the peak value of the voltage between terminals A and B. In practice, voltage VZ is imposed to obtain a sufficient discharge voltage and the maximum tolerable value of resistor Rs must be relatively high to allow the selection of a long time duration between two sparks.
Assuming that Rs=10 k.OMEGA., the peak voltage Vp between terminals A and B is 300 volts, and further assuming that VZ=250 volts, one obtains: EQU I.sub.B0(MAX) =(300-250)/10000=5 mA.
In practice, this is a very low triggering current for a thyristor. Usually, a conventional thyristor withstanding 400 volts has a triggering current I.sub.B0 of a few tens of a mA. A further difficulty is that voltage VZ of the avalanche diode Z should be accurately determined so that value I.sub.B0(MAX) does not vary too much and so that the triggering range is not further reduced.
FIG. 3 represents the voltage-current curve of a thyristor. When voltage V.sub.B0 (i.e., voltage VZ of the avalanche diode) is reached, the current in the thyristor starts to increase, then the voltage across the thyristor abruptly drops whereas the thyristor is turned on as soon as the current in the thyristor has reached I.sub.B0. The invention aims at providing a dipole component of the thyristor-type having both a low value I.sub.B0 and an accurately determined value V.sub.B0 (VZ).
FIG. 4 represents a conventional circuit of a breakover triggered dipole component including an amplifying-gate thyristor circuit providing this function. The thyristor Th is associated with a pilot thyristor Th1. The anodes of thyristors Th and Th1 are interconnected, the cathode of thyristor Th1 is connected to the gate of thyristor Th1 through a resistor R. The triggering diode Z is disposed between the anode and the gate of thyristor Th1. The gate-cathode resistor of thyristor Th is labeled R'.
Techniques are known to fabricate a very sensitive thyristor Th1 but this requires the use, between the cathode and the gate, of a resistor R having a non-negligible value (ranging approximately from 1 to 10 k.OMEGA.) to prevent the thyristor Th1 from being triggered by spurious overvoltages on the supply, i.e., by dV/dt triggering.
FIG. 5 illustrates a conventional monolithic component implementing the circuit of FIG. 4. This component is fabricated from a low doped N-type substrate 21. In the upper surface of the substrate, P-type regions 22, 23 and 24 correspond to the anode of diode D, to the gate of thyristor Th and to the gate of the pilot thyristor Th1, respectively. N-type regions 26 and 27 correspond to the cathodes of thyristors Th and Th1, respectively. The cathode of thyristor Th is provided with emitter-shorts to render this thyristor non-sensitive whereas the cathode of the pilot thyristor Th1 is devoid of emitter-shorts to render the pilot thyristor highly sensitive. The rear surface of the substrate includes, facing the cathodes of thyristors Th and Th1, a P-type region 28 corresponding to the common anode of thyristors Th and Th1. The rear surface further includes, facing the anode region 22 of diode D, an N.sup.+ -type region 29 corresponding to the cathode contact of diode D. The rear surface is uniformly coated with a metallization 30. The cathode area of thyristor Th and the anode area of diode D are coated with a metallization 31. The cathode region 27 of thyristor Th1 is connected to the gate region 23 of thyristor Th through a metallization 32. Resistor R is formed by a low conductive P-type region 34 that is disposed between the P regions 23 and 24. The junction corresponding to the zener diode Z is formed by providing a highly doped N-type layer 35 at the interface between region 24 and substrate 21.
The implementation illustrated in FIG. 5 has several drawbacks. On the one hand, this circuit requires, in addition to the conventional layers used for the fabrication of a thyristor, the presence of the highly doped N-type "buried" layer 35 and more particularly the presence of the low-doped P-type region 34 which imposes the provision of additional fabrication steps. On the other hand, as indicated above, in order to obtain a sufficient resistance, approximately 1 to 10 k.OMEGA., between the cathode and the gate of thyristor Th1, region 34 should have a very low doping level. This imposes fabrication constraints and makes the device difficult to reproduce from one batch to the other.